TopBand: Light Dimmer Noise

[w8jitom@postoffice.worldnet.att.net]

> From:          owens@stout.atd.ucar.edu (Chip Owens)

Hi Chip,
> I have tried shielded loops, both horizontally and
> vertically polarized. These don't help much.

The reason they don't help is if you shield an antenna from the 
time varying electric field you also shield it for any time varying 
magnetic fields or radiation fields. All shielding does is make the 
loop's "shield conductor" become the actual antenna, instead of the 
stuff inside the shield. 

Same with a noise source. It contains both electric and magnetic 
induction fields, as well as a radiation field.

> The noise blanker in my OMNI-V helps some, but still
> the average noise level hovers around the S-7 level.

Noise blankers only work on sharply rising pulses with short duty 
cycles. Light dimmer (and computer noises) are more like regular 
carriers with buzzy modulation.

> Has anyone had any success in minimizing the effects of
> light dimmer noise using the DSP boxes? or by other
> means?

Brian Beasly had an article in QST about low phased dipoles. If you 
copy it and make sure balance is maintained, it might help. It was 
one of the best ideas I've seen in print. 

DSP boxes help, especially ones with multiple manual filters, but 
the best solution (other than turning off the source) is to use a 
phasing system with a sense antenna to cancel the noise. I've been 
using a variable phase combiner for many years now, and MFJ just 
copied the design for production. It's surface mount machine 
assembled so QC should be excellent. It has good dynamic range, low 
noise, nearly perfect phase rotation, and excellent amplitude 
flatness. I have used several boxes like this to combine several 
antennas nulling noise or peaking signals from several sources at 
once for years now.

But even a good noise canceller will have restrictions. It 
either requires the signal come from ONE source, or if you have 
multiple sources of noise the canceler will work ONLY if they are all 
from the same basic direction and all noise sources arrive at both 
the sense and main antennas with the same signal level ratios. If 
the sources fit these requirements, the ultimate attenuation is 
near infinite!!!

Other than removing the dimmers or phasing antennas, your 
probably stuck. Shields won't do the trick at all.
73, Tom W8JI 

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